Active Sonar (SOund Navigation And Ranging) is a technique whereby acoustic signals are emitted by an acoustic source device to propagate underwater, reflect or scatter from various underwater objects or bathymetric features, and be detected by an acoustic receiver. Position and velocity of a submerged object can be estimated from intensity, timing, phase, Doppler shift, directionality, or other properties of the acoustic signals reflected or scattered from the object and then detected. Sources and receivers can be ship-borne, submarine-borne, borne by a floating buoy (referred to as a sonobuoy), or fixed to (or resting upon) the sea floor or other bathymetric feature (e.g., a seamount or guyot).
The simplest active sonar systems are monostatic and non-coherent, i.e., a single device is the source of a simple impulsive acoustic signal (such as an explosion) and also receives back-reflected or back-scattered echoes of that signal. Bistatic systems employ source and receiver at separate locations, while multistatic systems employ multiple sources and/or multiple receivers at multiple locations. A larger number of sources and/or receivers provides a correspondingly larger dataset with which to detect a submerged target object and from which to extract estimates of the position and velocity of the target object. Target position and velocity can be estimated from the received signals using any suitable computation technique, e.g., using one or more techniques such as Sequential Bound Estimation (SBE; disclosed in U.S. Pat. Nos. 7,219,032, 7,363,191, 8,010,314, 8,311,773, and 8,639,469, each issued in the name of inventor John Louis Spiesberger) and each incorporated by reference as if fully set forth herein).
More complex active sonar systems are so-called coherent systems, in which the acoustic signals emitted by the source(s) have more complex temporal or frequency characteristics (e.g., frequency chirp, multiple pulses, varying pulse intervals or durations, or other varied or optimized waveform). Estimates of a target's velocity can be made with traditional Doppler estimation techniques (e.g., using matched filters) or with more accurate techniques such as Coherent Time Change Estimation (CTCE; as disclosed in (i) Spiesberger et al, J. Physical Oceanogr., 19(8), 1073-1090, 1989; (ii) Headrick et al, J. Acoust. Soc. Am., 93, 790-802, 1993; and (iii) Birdsall et al, J. Acoust. Soc. Am. 96, 2343-2352, 1994, each of which is incorporated by reference as if fully set forth herein) for the bistatic or multistatic Doppler estimation technique disclosed in the second Appendix of the provisional application cited and incorporated above.
Sonar has been and still is used extensively by naval forces in submarine and anti-submarine warfare (ASW) for detecting, locating, and targeting submerged enemy submarines. The importance of rapidly obtaining accurate estimates of position and velocity of an enemy submarine during a battle or other tactical situation is obvious. A common approach is deployment of a set of multiple sonobuoys in an area where an enemy sub is suspected and operation of those sonobuoys as a multistatic active coherent (MAC) system to estimate the target sub's position and velocity. A central controller (typically mounted in an aircraft such as a Boeing P8A Poseidon deployed in the vicinity; often the same aircraft that dropped the sonobuoys) directs and monitors the emission of acoustic signals by the source sonobuoy(s) and receives from the receiver sonobuoy(s) electronic signals representative of the received acoustic signals (or filtered or otherwise processed versions thereof). Calculations (using any one or more suitable computation techniques, e.g., CTCE or SBE) based at least in part on those received waveforms (using as inputs position and velocity information for each of the source and receiver sonobuoys, and in some cases also using source timing and waveform information) yields an estimate of the position and velocity of the submerged target sub. SBE has been employed previously for naval target estimation. CTCE, disclosed in the provisional application cited and incorporated above, has not been known to be employed to detect or locate target objects of interest to the Navy.
Detection of a target object, and accuracy of any estimated target position and velocity derived from signals emitted and received in a MAC system, depend on the accuracy within which the positions and velocities of the multiple source and receiver sonobuoys is known. Once deployed and floating at the sea surface, the sonobuoys drift, each one independent of the others. The sonobuoys can be equipped with Global Positioning System (GPS) or similar equipment so as to enable each to determine its position and velocity, and then the sonobuoys can transmit that information to the central controller for use in subsequent processing. It would be desirable to develop systems and methods that enable earlier detection of target objects and accurate target position and velocity estimates using multiple-sonobuoys of a MAC sonar system even in the absence of GPS signals or other extrinsic positioning signals acquired for a sonobuoy after its initial deployment. Initial estimates of sonobuoy positions are typically made using navigation systems on the deploying aircraft and models of the sonobuoys' trajectories in the air after release until hitting the water. However, once the sonobuoys start to drift, uncertainty of their respective positions grows with time in the absence of GPS or other extrinsic positioning signals. Position uncertainty of drifting sonobuoys has been, and continues to be, a fundamental limitation on the goal of reliably detecting and accurately locating target objects using a MAC system. The United States Navy has solicited proposals for such systems and methods for decades and continues to desire further improvements.